Zombie units will be separated into their specific roles(in fact this will probably be done for all units, but there are always exceptions :P)
Basic Infantry-
Rotters- Typical slow almost brain dead zombies, just add guns.
Virals- Slightly weaker than the Rotters, Viral Zombies are able to replenish their number, each unit they kill turning into another Viral within a short period of time. Virals tend to be an off-purple color, with blotches of deeper purples.
Bloodstockers- More fragile than the other types of zombies, Bloodstockers are slightly faster and explode in diseased blood upon death, making them the ultimate kamikazes. Bloodstockers are somewhat bloated and almost totally red, never carrying more than a knife on them.

Fast Attack-
Reavers- One of the more dangerous forms to come from the cesspools that spawn them, Reavers are designed purely on speed and strength.  Their long lanky arms and legs end in horrible 3 foot long claws and their bodies are made up entirely of sinew and muscle, lacking the skin of the other dead.
Rotten Bikers- Essentially just Rotters on motorcycles, Rotten Bikers are actually somewhat intelligent, showing a penchant for attaching large blades and spikes to their motorcycles and riding amidst their foes with reckless abandon, steering with one hand and firing wildly with the other.
Chimerics- Chimerics are massive piles of the dead sewn and grafted together to create a horrific mockery of cavalry, their 4 legged bodies rushing forward to break human ranks with chainsaws and the like.
Heavy Attack-
Bloodhulks- Bloodhulks are immense versions of the Bloodstockers, sacrificing a large portion of their speed for enhanced durability and strength. Most bloodhulks display a very rudimentary intelligence, slamming their massive club like arms into buildings to collapse them, flipping vehicles, and finally lifting their enormous stomach like bloodsacks and slamming them on their foes. Like their brethren Bloodhulks are filled with diseased blood, but unlike bloodstockers they vomit the blood on their foes instead of exploding when they die.